US5081069A - Method for depositing a Tio2 layer using a periodic and simultaneous tilting and rotating platform motion - Google Patents

Abstract

Method and apparatus are disclosed for depositing a uniform layer of material, such as titanium dioxide, on the surface of an object, such as a silicon sphere of a solar array (7). Component gases are injected at predetermined rates into a heated reaction chamber (5) where they react. Because of the reaction rate and injection velocities of the gases, the reaction is substantially completed at a calculated location inside the reaction chamber (5). The object which is to receive the layer, such as the solar array (7), is placed at the calculated location in the reaction chamber (5). The platform (68) to which the solar array (7) is attached is simultaneously tilted and rotated such that all areas of the surface of the array (7) are uniformly exposed to the titanium dioxide reactant.

Description

RELATED APPLICATIONS

This application is related to co-pending applications U.S. Ser. No. 387,677, filed July 31, 1989; U.S. Ser. No. 387,250, filed July 31, 1989; Ser. No. 388,105, filed July 31, 1989; U.S. Ser. No. 388,280, filed July 31, 1989; U.S. Ser. No. 387,929, filed July 31, 1989; and U.S. Ser. No. 387,244, filed July 31, 1989, incorporated by reference herein.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to the deposition of a layer of material on the surface of an object, and more particularly to a method and apparatus for depositing a layer on a spherical object.

BACKGROUND OF THE INVENTION

In solar cell design, it is important that an anti-reflective layer be applied to the solar cell to increase its current output and efficiency. When the surface of the solar cell is flat, it is relatively easy to uniformly deposit a layer of anti-reflective material over a large surface area. Various techniques, such as spin-on of solutions, chemical vapor deposition (CVD) and sputter deposition, have been used to deposit a layer on flat surfaces.

Recently, solar cells have been built by incorporating a plurality of silicon spheres to form a solar array which efficiently produces electricity from the sun's radiation as described in U.S. Pat. No. 4,691,076 to Levine et al., incorporated by reference herein. During production of the array, each silicon sphere is embedded in aluminum foil such that only a hemispherical surface is exposed. Unfortunately, none of the prior layering techniques produced the desired thin, uniform layer on the hemispherical surfaces of the silicon spheres used in these new solar cells.

When performing the spin-on technique, a titanium containing solution is spun onto a flat surface to form a thin layer of titanium dioxide to serve as the anti-reflective layer. However, when depositing a layer on a curved surface using the spin-on technique uniform coverage and thickness on the hemispherical surfaces of the silicon cells is difficult to obtain.

During sputtering deposition, a titanium metal is sputtered onto the desired surface and then oxidized to form titanium dioxide. However, oxidation of the metal requires high temperatures which may damage or destroy the material of the solar array.

Depositing layers by CVD methods on spherical surfaces also does not result in uniform thickness and coverage when applied to irregular or curved surfaces, such as the hemispherical surfaces of the spherical solar cells. Such deposition is anisotropic such that only those areas of the solar cell surface which are approximately perpendicular to the flow of the reacting chemicals obtain an adequate layer. Other areas of the surface, such as those on the sides of the silicon spheres, are approximately parallel to the flow of the reacting chemicals and obtain an inadequate deposit.

Therefore, a need has arisen to provide an apparatus and method for depositing a uniform layer of a titanium dioxide compound onto silicon spheres used in solar cells.

SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus and method are provided which substantially eliminate the disadvantages and problems associated with prior methods of depositing a layer of material, such as titanium dioxide, on the surface of spherical objects.

In one aspect of the invention, a mixing chamber is provided to allow a first set of gases to mix, such as titanium isoproxide (TIP) vapor and oxygen. The mixture of the first gases is injected into a reaction chamber as is a second gas, such as water vapor. The object on which the location inside the reaction chamber such that the reaction producing the layer substantially occurs on the surface of the object.

In another aspect of the present invention, the object in the reaction chamber is attached to a platform which is simultaneously and periodically tilted and rotated. Thus, all portions of the surface of the object are exposed equally to the reacting chemicals. In this fashion, the surface of an irregular or spherical object, as well as of a flat object, can be provided with a uniform layer of material.

Hence, the apparatus and method of the present invention provide the technical advantage of depositing a uniform layer of material on the surface of an object, including, but not limited to, surfaces which are spherical.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of the present invention;

FIG. 2 illustrates a detailed schematic representation of the present invention;

FIG. 3 illustrates a sectional view of the barrier wall between the mixing and reaction chambers on the line II--II in FIG. 2;

FIG. 4 illustrates a sectional view of the end wall of the mixing chamber on the line III--III in FIG. 2;

FIG. 5 illustrates a detailed view of the solar array platform held by the rotation tube and push rod;

FIG. 6 illustrates a detailed view of the drive mechanisms for tilting and rotating the platform; and

FIG. 7 illustrates an alternative embodiment of the drive mechanisms.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention is best understood by referring to FIGS. 1-7 of the drawings, like numerals being used for like and corresponding parts of the various drawings.

FIG. 1 illustrates a block diagram of the apparatus of the present invention, indicated generally as 1. Twogas sources 2 and 3 are coupled to amixing chamber 4.Mixing chamber 4 is coupled to areaction chamber 5. Agas source 6 is also coupled toreaction chamber 5. Asolar array 7, or other surface to be coated with a layer of material, is placed insidereaction chamber 5.

In operation,gas sources 2 and 3 inject two gases intomixing chamber 4 where the gases mix to form a third gas which is injected intoreaction chamber 5.Gas source 6 injects a fourth gas intoreaction chamber 5 in the same direction as the third gas is injected. The third and fourth gases do not react instantaneously, but rather react at a known rate. Because of the injection velocities of the gases, the reaction is substantially completed at a location within reaction chamber 5 a calculated distance from the points at which the gases are injected.Solar array 7 is positioned inreaction chamber 5 at the calculated location such that the reaction is substantially completed at the surface ofsolar array 7.Solar array 7 is simultaneously tilted and rotated withinreaction chamber 5 thereby exposing all areas of the surface ofsolar array 7 to the reacting gases. Consequently, the reactant, an antireflective material, is deposited in a uniform layer on the surface ofsolar array 7.

In one aspect of the present invention, the first and second gases are oxygen and titanium isoproxide vapor, respectively, and the third gas is a mixture of the two. The fourth gas is water vapor and the antireflective reactant is titanium dioxide.

FIG. 2 shows more detailed a schematic representation of the apparatus of the present invention

Mixing chamber 4 andreaction chamber 5 are placed end-to-end in a furnace 16 and are separated by abarrier wall 18. Anend wall 20 ofmixing chamber 4seals mixing chamber 4 from the outside environment. Twoholes 22 and 24 formed inend wall 20 serve as input ports to mixingchamber 4. Anadditional hole 26 formed inend wall 20, permits aninlet line 28 to pass throughmixing chamber 4 and intoreaction chamber 5. Aninjection port 30 formed inbarrier wall 18 between the twochambers 4 and 5 allows the chemical mixture formed in mixingchamber 4 to pass intoreaction chamber 5. In addition,inlet line 28 passes throughinjection port 30 intoreaction chamber 5.

Oxygen is supplied bygas source 2 through avalve 34 coupled to atube 36.Tube 36 terminates atinput port 22 inend wall 20 of mixingchamber 4.

A titanium isoproxide (TIP) solution is held in agas source 3 to be heated by aheater 40. One end oftube 42 is submerged in the TIP solution; the other end oftube 42 is connected to avalve 44 which is coupled to anothertube 46 which terminates atnitrogen gas tank 48. One end of anothertube 50 is situated above the TIP solution. The other end oftube 50 is coupled totube 52 which terminates atinput port 24 inend wall 20 of mixingchamber 4.Tube 52 is also coupled tonitrogen tube 46 throughvalve 54.

Similarly, atube 56 is submerged in deionized water held in agas source 6.Tube 56 is coupled tonitrogen tube 46 throughvalve 60. One end of anothertube 62 is situated above the surface of the water. The other end oftube 62 is coupled toinlet line 28 which terminates inreaction chamber 5.Inlet line 28 is also coupled tonitrogen tube 46 throughvalve 64.

Solar array 7 is clipped onto one side ofplatform 68.Platform 68 is coupled on the other side torotation tube 70 with twopivots 72.Rotation tube 70 is held in place by bearingunits 74 attached tobase 76. A cog-belt drive, indicated generally at 78, is placed circumferentially aroundrotation tube 70 and is coupled to a motor (not shown).

Pushrod 80 is also coupled toplatform 68 withpivot 82. Pushrod 80 is situated insiderotation tube 70 and the end oppositeplatform 68 is coupled tocam 84 with ball joint 86.Cam 84 is coupled to ashaft 88 ofmotor 90 which is attached tobase 76.

A set ofwheels 92 attached to base 76 allowplatform 68 andsolar array 7 to be easily pushed into and pulled out ofreaction chamber 5.

FIG. 3 illustrates a view ofbarrier wall 18 between mixingchamber 4 andreaction chamber 5.Opening 30 is shown in the center ofbarrier wall 18 withtube 28 passing throughopening 30.

FIG. 4 illustrates a view ofend wall 20 of mixingchamber 4 and shows the locations ofinjection ports 22 and 24 andopening 26 through whichinlet line 28 passes.

FIG. 5 illustrates the details of thepivots connecting platform 68 withrotation tube 70 and pushrod 80.Solar array 7 is clipped to the side ofplatform 68 opposite the side shown in FIG. 5. Opposite sides of the end ofrotation tube 70 have been cut away as indicated and pins 93 are inserted throughattachment plates 94 and through holes (not shown) formed in the end ofrotation tube 70. Asimilar pivot 82 provides the attachment forpush rod 80 toplatform 68. Pushrod 80 is placed between twoattachment plates 96 andpin 98 is inserted through holes formed inattachment plates 96 and pushrod 80.Pivot 82 is located a predetermined distance from an imaginary line connecting pivot points 72.

FIG. 6 illustrates the details of the opposite end ofrotation tube 70 showing the drive mechanisms. The end ofrotation tube 70 not connected toplatform 68 is closed by acap 100 in the center of which has been drilled a hole. Pushrod 80, located inside ofrotation tube 70, extends out through the hole inend cap 100. Coupling 102 has a sleeve on one end into which the end ofpush rod 80 is inserted and fixed. The other end ofcoupling 102 encircles ball joint 86 in such way that ball joint 86 can freely rotate. Ball joint 86 is attached to one side of cam 84 a predetermined distance from the point at whichcam 84 is attached toshaft 88 ofdrive motor 90.

Toothed sprocket wheel 104 is secured around the circumference ofrotation tube 70. A similar, but smaller,toothed sprocket wheel 106 is secured to a shaft 08 ofrotation drive motor 110. Acog belt 112 engages the sprockets ofsprocket wheels 104 and 106 to transfer the rotational motion fromrotation drive motor 110 torotation tube 70. In one aspect of the present invention,small sprocket wheel 106 may be coupled to drivemotor 110 through a set of reduction gears andcam 4 may be coupled to drivemotor 90 through a separate set of reduction gears. Both drivemotors 90 and 110 are mounted onbase 76.

FIG. 7 illustrates an alternative embodiment forpush rod 80 drive mechanism. As before, drivemotor 90 is mounted onbase 76. Acam 114 is attached off-centered tomotor shaft 88. Extending from the end ofrotation tube 70 is ahousing 116 through which pushrod 80 extends. Acoupling 118 contains a sleeve at one end through which pushrod 80 is inserted and fixed. The opposite end ofcoupling 118 contains an axle on which a cam follower 120 is mounted. A spring mechanism (not shown) insidehousing 116 biases cam follower 120 to maintain contact withcam 114 at all times.

Alternative means for drivingrotation tube 70 and pushrod 80 are possible without deviating from the scope of this description or the claims.

Referring again to FIG. 2, in operation, the TIP solution is heated ingas source 3 byheater 40. Nitrogen fromnitrogen tank 48 flows throughtube 42 and bubbles through the TIP solution to pick up TIP vapors.Valve 44 controls the rate at which the nitrogen bubbles through the TIP solution. Simultaneously, nitrogen controlled byvalve 54 sweeps the TIP vapors throughtube 50 and intotube 52 to be injected into mixingchamber 4 throughinjection port 24.

In a similar manner, deionized water ingas source 6 is swept by nitrogen gas intoinlet line 28 and injected intoreaction chamber 5.

Oxygen fromgas source 2 flows throughtube 36 directly into mixingchamber 4 throughinput port 22.

The TIP is injected throughinput port 24 at a rate determined by the setting ofnitrogen sweep valve 54. The rate at which oxygen is injected into mixingchamber 4 throughinput port 22 is determined by the setting ofvalve 34. The rate at which water vapor is injected intoreaction chamber 5 is determined by the setting ofvalve 64.

The TIP and the oxygen are mixed in mixingchamber 4 and are injected throughinjection port 30 intoreaction chamber 5. The rate at which the TIP and oxygenenter mixing chamber 4 throughinput ports 24 and 22, respectively, determines the rate at which the mixture of the two substances entersreaction chamber 5 throughinjection port 30.

Onceinside reaction chamber 5, the TIP and water react to form titanium dioxide as follows: ##STR1##

In another embodiment, a straight chained proxide can be used to carry the titanium. However, reaction temperatures would be required which are higher than the temperatures required by the preferred embodiment.

The reaction in which the titanium dioxide is produced occurs at a known rate. Because the TIP/oxygen mixture and the water vapor are injected intoreaction chamber 5 with a known velocity, the reaction rate can be translated into a distance such that the reaction is substantially completed a certain distance frominjection port 30. To obtain a proper layer of titanium dioxide onsolar array 7,solar array 7 must be placed at the location inreaction chamber 5 where the chemical reaction is substantially completed. Knowing the reaction rate and the velocities with which the TIP/oxygen mixture and water vapor are injected intoreaction chamber 5, the optimum distance ofarray 7 frominjection port 30 can be calculated.

In order for all areas of the exposed spherical surfaces ofsolar array 7 to be uniformly covered with the titanium dioxide layer,array 7 must be simultaneously rotated and tilted, thereby presenting all sides of each hemisphere to the reacting chemicals.

Rotation drive motor 110 is electrically connected to a variable speed control (not shown) which is adjusted for the desired rotation speed.Rotation drive motor 110 causessmall sprocket wheel 106 to turn.Small sprocket wheel 106 drivescog belt 112 which, in turn, driveslarge sprocket wheel 104 causingrotation tube 70 andplatform 68 to rotate.

Simultaneously, pushrod drive motor 90, which is also electrically coupled to a variable speed control (not shown) causescam 84 to rotate. Ball joint 86, which is fixed tocam 84, revolves about the point at whichshaft 88 is attached tocam 84. Ball joint 86 also rotates within the enclosure at one end ofcoupling 102, thereby translating the rotational motion ofdrive motor 90 into a reciprocal linear motion forpush rod 80.

Aspush rod 80 reciprocates throughrotation tube 70, it alternately pushes and pulls onplatform 68 atpivot 82.Platform 68, in turn, oscillates through a prescribed arc on pivots 72. The arc through whichplatform 68 oscillates is determined by two factors: (1) the shortest distance between thepivot 82 and an imaginary line between the twopivots 72; and (2) the distance between ball joint 86 oncam 84 and the point oncam 84 at whichshaft 88 is attached.

In the alternative embodiment of thepush rod 80 drive mechanism shown in FIG. 7, drivemotor 90 causescam 114 to revolve in an eccentric fashion about the point at whichshaft 88 is attached. As mentioned, the spring mechanism insidehousing 116 biases cam follower 120 againstcam 114. Consequently, ascam 114 rotates, the eccentric motion ofcam 114 is translated into a reciprocal lateral motion ofpush rod 80. The rotation tube drive mechanism remains the same as depicted in FIG. 6.

EXAMPLE

Tests have been made with device 1 to deposit a layer of titanium dioxide on spherical solar cells in an array. The parameters which result in a uniform deposition are now described.

The TIP is heated to approximately 100° C., the deionized water is maintained at room temperature (approximately 25° C.) and the reaction chamber is heated by the furnace to between approximately 200° C. and approximately 300° C.

The oxygen is injected into the mixing chamber at the rate of approximately 3,000 cc/min. The nitrogen is bubbled through the TIP solution at a the rate of approximately 200 cc/min. The nitrogen sweep of the TIP vapors into the mixing chamber occurs at the rate of approximately 2,000 cc/min.

The nitrogen is bubbled through the deionized water at the rate of approximately 20 cc/min. and the nitrogen sweep of the water vapors into the reaction chamber is approximately 1,000 cc/min.

In the prototype tested, the mixing chamber and reaction chamber are each 21/2 inches in diameter and the distance between the solar array and the injection port in the reaction chamber averages 51/2 inches. The injection port in the center of the barrier wall between the mixing and reaction chambers (through which the TIP/oxygen mixture is propelled) is 1/2 inch. The inlet line through which the water vapor is injected into the reaction chamber has an inside diameter of 6 mm.

The rotation tube is rotated at the rate of four revolutions per minute and the platform on which the array is clipped tilts at the rate of four oscillations per minute. The solar array has a total area of approximately ten cubic centimeters and comprises spherical solar cells, each with a diameter of 17.5 mils. The solar cells are mounted in aluminum foil on 19 mil centers; only hemispherical surfaces are exposed for coating.

Given these parameters, titanium dioxide is deposited on the solar cells at the rate of approximately 150 angstroms per minute until the layer attains a desired thickness of between approximately 700 and approximately 800 angstroms. After the deposition has been completed, the array is removed from the platform and sintered at approximately 400° C. to complete the process.

The resulting layer creates an anti-reflective coating on the solar cells having a density of approximately 2.54 and refractive index of approximately 2.4. This compares with a refractive index of less than approximately 2.1 for prior art CVD methods which indicates that less light incident upon the solar cells will be reflected and more light will be absorbed and converted into electrical current.

Thus, the apparatus and method of the present invention have the technical advantage of providing uniform deposition of titanium dioxide on spherical solar cells at far lower temperatures than are required by the prior art. In the tests, the resulting anti-reflective layer has increased the short-circuit current of the solar cells by between 20%-30%.

Layers of titanium dioxide can also be deposited on irregular and flat surfaces using the apparatus and method described and claimed herein. Furthermore, layers of other metal oxides can be deposited using the apparatus and method of the present invention.

Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (19)

What is claimed is:

1. A method for depositing a layer of material on the surface of an object, comprising the steps of:

removably holding the object on a platform;

placing said platform at a predetermined location within said reaction chamber;

imparting a rotating motion to a rotation tube coupled to said platform;

imparting a periodic, reciprocal linear motion to a push rod coupled to said platform such that the platform periodically and simultaneously tilts and rotates whereby all areas of the surface of the object are uniformly exposed to the reacting gases;

injecting a first gas into a reaction chamber at a predetermined rate; and

injecting a second gas into said reaction chamber at a predetermined rate to react with said first gas at a predetermined location within said reaction chamber.

2. The method of claim 1 and further comprising the step of mixing a third and a fourth gas to generate said second gas.

3. The method of claim 1 wherein said first gas is water vapor.

4. The method of claim 1 wherein said second gas is titanium isoproxide and oxygen.

5. The method of claim 2 wherein said third gas is titanium isoproxide.

6. The method of claim 2 wherein said fourth gas is oxygen.

7. The method of claim 1 wherein said step of imparting a rotating motion to said rotation tube comprises the step of driving said rotation tube with a motor.

8. The method of claim 1 wherein said step of imparting a reciprocating linear motion to said push rod comprises the step of driving a cam coupled to said push rod with a motor.

9. A method for depositing a layer of material on the surface of an object, comprising the steps of:

injecting a first gas into a reaction chamber at a predetermined rate;

injecting a second gas into said reaction chamber at a predetermined rate to react with said first gas at a predetermined location within said reaction chamber;

mixing a third and a fourth gas to generate said second gas;

removably holding the object on a platform;

placing said platform at a predetermined location within said reaction chamber;

imparting a rotating motion to a rotation tube coupled to said platform; and

imparting a periodic, reciprocal linear motion to a push rod coupled to said platform such that the platform periodically and simultaneously tilts and rotates whereby all areas of the surface of the object are uniformly exposed to the reacting gases.

10. The method of claim 9 wherein said step of imparting a rotating motion to said rotation tube comprises the step of driving said rotation tube with a motor.

11. The method of claim 9 wherein said step of imparting a reciprocating linear motion to said push rod comprises the step of driving a cam coupled to said push rod with a motor.

12. The method of claim 9 wherein said first gas is water vapor.

13. The method of claim 9 wherein said second gas is titanium isoproxide.

14. The method of claim 9 wherein said third gas is titanium isoproxide.

15. The method of claim 9 wherein said third gas is oxygen.

16. A method for depositing a uniform layer of titanium dioxide on the surface of a solar cell array, comprising the steps of:

heating a solution of titanium isoproxide to produce titanium isoproxide vapors;

bubbling nitrogen gas through said titanium isoproxide solution to pick u said titanium isoproxide vapors;

sweeping said titanium isoproxide vapors into a mixing chamber with nitrogen gas;

injecting oxygen gas into said mixing chamber to mix with said titanium isoproxide vapors;

injecting the mixture of titanium isoproxide vapors and oxygen into a reaction chamber;

bubbling nitrogen gas through deionized water to produce water vapor;

sweeping and injecting said water vapor into said reaction chamber;

heating said reaction chamber;

producing a reactant of titanium dioxide;

simultaneously and periodically tilting and rotating the solar cell in said reaction chamber; and

depositing said reactant on the solar cell array.

17. The method of claim 16 wherein said tilting and rotating step comprises the steps of:

removably attaching the solar cell array to a platform;

imparting a rotating motion to a rotation tube coupled to said platform; and

imparting a periodic, reciprocal linear motion to a push rod coupled to said platform such that the platform periodically and simultaneously tilts and rotates whereby all areas of the surface of the solar cell array are uniformly exposed to the titanium dioxide reactant.

18. The method of claim 16 wherein said titanium proxide is heated to between 85° C. and 110° C.

19. The method of claim 16 wherein said reaction chamber is heated to between 200° C. and 300° C.

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